In general, an internal combustion engine is provided with an oil pump to supply oil from an oil pan arranged at a bottom part of the engine to each mechanical portion thereof arranged at the upper side. In most cases, a trochoid type oil pump (trochoid pump) is used for a four-stroke engine mounted on, for example, a motorcycle, an outboard engine, a snowmobile, or the like (e.g., see Patent Document 1). In some cases, a trochoid pump is used for supplying oil to a transmission or the like.
FIG. 8 illustrates an oil passage using a trochoid pump. As illustrated in FIG. 8, a trochoid pump 102 sucks oil stored in an oil pan 101 arranged at a bottom part of an engine through a suction port and pressurizes and discharges the oil through a discharge port. The oil discharged from the trochoid pump 102 is supplied to a variety of respective mechanical portions 104 through an oil filter 103. Then, the oil is returned to the oil pan 101 from the respective mechanical portions 104.
FIGS. 9(a)-9(e) are views illustrating operation of the trochoid pump 102. FIGS. 9(a)-9(e) are disclosed as FIG. 3 in Patent Document 1. FIGS. 9(a)-9(e) illustrate, for a single pump chamber, a sucking and compressing stroke of air-mixed oil, an ejecting stroke of air and a part of oil, and a discharging stroke of oil. Here, regions filled with oil are illustrated with slashes.
First, when an inner rotor 13 and an outer rotor 12 are rotated clockwise, oil starts to be sucked through a suction port 11b as illustrated in FIG. 9(a). Then, when the inner rotor 13 and the outer rotor 12 are further rotated clockwise, oil is further sucked as illustrated in FIG. 9(b).
Next, the air ejecting stroke starts from a state in which oil is sucked at a maximum as illustrated in FIG. 9(c). Accordingly, as illustrated in FIG. 9(d), the pump chamber starts to communicate with an ejection port 11d, and a part of air-mixed oil is ejected from the ejection port 11d through a passage 11d′. 
When the inner rotor 13 and the outer rotor 12 are further rotated clockwise, the ejection port 11d is closed and the discharging stroke starts. In the discharging stroke, as illustrated in FIG. 9(e), remaining oil is discharged through a discharge port 11c and pressure-fed toward the variety of respective mechanical portions 104.
Here, as illustrated in FIG. 9(c), the maximum volume of oil to be discharged through the discharge port 11c corresponds to a region of oil S compressed in the previous stroke. Such a technology to eject air mixed in oil by arranging an ejection port that communicates with the outside of a pump is also disclosed, for example, in Patent Document 2.
Patent Document 1: Japanese Patent Application Laid-Open No. 2011-231772
Patent Document 2: Japanese Patent Application Laid-Open No. H9-203308
As disclosed in Patent Document 1 and Patent Document 2, in a conventional trochoid pump, an air ejection port is arrange between a suction port and a discharge port to set an air ejecting stroke between a sucking stroke and a discharging stroke. Generally, in an internal gear pump such as a trochoid pump, oil and mixed air tend to be separated with the oil being at the outer side due to centrifugal force caused by rotation of an outer rotor and inner rotor and the mixed air being at the inner side. Therefore, an air ejection effect can be enhanced by arranging an air ejection port at the inner side.
However, if an air ejection port is arranged large simply at the inner side, the air ejection port communicates with the suction port and air is sucked with negative suction pressure through the air ejection port. Alternatively, the air ejection port communicates with the discharge port and discharge pressure leaks to the air ejection port. Thus, when the air ejection port communicates with either the suction port or the discharge port, a desired amount of oil cannot be sucked and discharged at desired pressure resulting in pumping function deterioration. Therefore, an air ejection port cannot be arranged large simply at the inner side.
As described above, since an air ejection port is required to be arranged at a limited space between a suction port and a discharge port, it has been difficult to ensure port area thereof. Accordingly, there has been a problem that an air ejection effect is difficult to be enhanced with small port area. For some applications, there may be a case that an ejection rate of air-contained oil is required to be a given value or higher. Then, there has been a case that port area cannot be ensured for actualizing the ejection rate of air-contained oil. In addition, such small port area of an air ejection port has been causing a problem that a torque required for rotating a rotor rotating shaft is increased due to enlarged ejection resistance.
To solve such problems, if an air ejection port 220 is designed to be excessively large at a position without having communication with either of a suction port 210 and an discharge port 230 as illustrated in FIG. 10, a pump chamber 240 of a previous stroke and a pump chamber 250 of a subsequent stroke communicate with each other through the air ejection port 220. (In FIG. 10, the rotors are illustrated to be rotated counterclockwise.) As a result, an ejection amount of air-contained oil ejected from the air ejection port 220 cannot be maintained at constant and the discharge amount and discharge pressure of oil fluctuate, resulting in causing a problem that a stable performance of the trochoid pump cannot be obtained.